Lol, I thought the question was going to be about bread too, until I read the post.
Goodness, I don't know. I didn't realize bun was such a recent term for the hairdo. I've certainly run across the term in historic fiction, but anachronisms are nothing new. I suppose one could get away with it in stuff set in an older period, because of the translation into modern English issue. But not if the story's set in the 1700 or 1800s before the term became commonplace.
Topknot? Or simply described as it appears: hair pulled back into a tight knot at the back of her head?
Or was this style even something people wore much before the term was coined? Sometimes terminology follows necessity, and we don't invent words for things we simply have little concept of. Adult women wore their hair braided or covered a lot prior to the mid to late1800s, didn't they? Or swept up into more elaborate styles?